Wednesday, 12 September 2007

Stuff that

I cooked some poultry stuffing and left it in a bowl in the fridge overnight covered with aluminium foil. In the morning there were holes in the foil where it had touched the stuffing, which was stained black under each hold. Uncooked stuffing does not produce this effect, and it makes no difference whether the stuffing is cooked inside the bird or separately. What is gong on here: and is the black stain poisonous?Andrew Stiller, Philadelphia, Pennsylvania, US

This sounds like something is disrupting the layer of aluminium oxide on the outer layer of the foil. This newly exposed pure aluminium then oxidises in the air, and this new oxide layer is disrupted again. This process continues until it has burned completely through the foil. The black residue is presumably an amalgam of the aluminium and whatever compound did the disrupting.

Several metals can cause this - for example, a droplet of mercury can eat its way through an aluminium plate overnight (described in a previous Last Word). This is why you're not allowed to take mercury thermometers on aircraft - if any mercury is spilled it'll slowly weaken whatever aluminium structures it encounters.

Given that you're still alive I can't imagine there's enough mercury in your stuffing to cause this. Perhaps the cooking process alters the oxidation state of some other metal compound in the stuffing, resulting in a form able to reduce aluminium oxide?

The following answer has been selected and edited by New Scientist staff

Without its submicroscopic insoluble skin of oxide, aluminium cookware would catch fire easily.

Normally, breaks in the oxide skin of aluminium heal instantly when the exposed metal reacts with say, air or water. But if, for example, mercury or certain alkalis or acids dissolve this skin, the exposed underlying metal reacts vigorously. So, while aluminium cookware and foil are safe and useful in the kitchen, it is important to keep them away from strong salt solutions or caustic soda, for example, and also from wet food when it is not actually cooking.

Wet, fatty materials such as cooked lard form fat-soluble detergents that penetrate microscopic chinks in the oxide layer, exclude air that otherwise would reseal the skin, and corrode pinholes into the metal. If floating fat has coated the metal, even cold chicken soup can eat through a thick aluminium pot overnight.

The black stain is mainly from small amounts of iron in the aluminium. It is not deadly, but it is better not to eat food contaminated with high levels of metals, which also spoil the taste. For wrapping cooked fatty or acidic food for more than short periods, plastic film is much better.

I'm not sure if this is correct, but I heard years ago that there was a link between aluminum ingestion and alzheimers.... Also, never make anything with tomotos in an aluminum pot... I slow cooked chili in an aluminum crock pot once... Tasted strongly metallic and badly damaged the metal's surface... For your question, i would agree with the fat landing on the aluminum theory.

The foil is reacting with either an acidic component of the stuffing or a sulphur compound in the stuffing. Either one will discolor or deteriorate the foil with holes. I use orange in my stuffing and cannot use foil because of the acid. I also use a high salt content water chestnut in the dressing that will desolve the foil and have a slight sulphur smell if left in contact with the foil. I suggest that you check your ingredients and see which one(s) are causing the damage to the foil.

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